Concrete or mortar is strong in compression but weak in traction. This weakness can give rise to cracks appearing in the concrete.
The process whereby concrete cracks comprises three stages, and these stages are always the same regardless of the mechanical load. Initially microcracking is observed which is diffuse and spread throughout the volume of the material. Thereafter, a certain number of the microcracks are seen to coalescence, thereby forming one or more macrocracks. Finally, during the last stage, one or more of the macrocracks created during the preceding stage propagate until the material ruptures completely.
In order to improve the strength of these materials in traction, proposals have been made to incorporate discontinuous fibers therein. Unlike the conventional reinforcement of reinforced concrete, these discontinuous fibers are distributed throughout the volume of the material.
The discontinuous fibers have an effect in all three stages of concrete cracking. During the first stage of cracking, the fibers act as stitches and slow down the growth of microcracks, thereby delaying the creation of macrocracks. Nevertheless, macrocracks do appear eventually, and the fibers then act on them as bridges conveying forces across the lips of the macrocracks, thereby ensuring that the cracked structure remains stable.
For the fiber to be effective at the scale of microcracks or at the scale of macrocracks, it is necessary for the fiber to be well anchored in the cement matrix.
In order to ensure that the fiber is well anchored, it must adhere well to the matrix. Adherence is a property of the fiber-matrix bond representative of the local resistance of the fiber to slipping. Obtaining good fiber adherence depends on the material from which the fiber is made, on its greater or lesser specific area, on its smoother or rougher surface appearance, or on the presence of crenellations, undulations, etc.
When adherence between the fiber and the matrix is not very good, it is nevertheless still possible to obtain adequate anchoring of the fiber. Under such circumstances, it suffices merely to ensure that fiber length is very long compared with the maximum crack gaps that the fiber is to bridge, or it is also possible to provide anchoring heads at the ends of the fiber, e.g. in the form of hooks.
Numerous designs of discontinuous metal fibers for improving the mechanical properties of a cement matrix are commercially available, however two main technological problems generally arise when discontinuous metal fibers are incorporated in a cement matrix.
The first problem is that the greater the quantity of fibers incorporated in the concrete, the more difficult the concrete becomes to handle, and this gives rise to problems in putting the fiber-containing concrete properly into place within shuttering. The solution for solving this problem consists in altering the granular skeleton of the concrete, i.e. the sand/aggregate ratio. The fiber-containing composite is then easier to handle, but its mechanical strength is reduced.
The second technological problem is that for most drawn steel wire fibers which are longer than the longest pieces of aggregate in the cement matrix, a fiber-tangling phenomenon occurs in the matrix, particularly when the percentage of incorporated fibers is high. The fibers tend to clump together. This phenomenon leads to a fiber composite being obtained in which there are spaces that include no fibers, thereby providing a material which is highly non-uniform. This feature is very detrimental to the mechanical strength of the composite.
Steel wire fibers are very ductile in traction, thereby enabling them to stretch considerably while continuing to withstand the force which is applied to them. For concrete, steel wire fibers should therefore be capable of stitching together the edges of macrocracks even when the gap width is large, while still imparting a degree of stability to the cracked structure.
Discontinuous straight fibers are known, but they suffer from poor anchoring in the matrix, and they tend to clump together in ordinary concrete whenever they are greater than or equal to 15 mm in length and for incorporated volume percentages that are greater than or equal to 1%.
Undulating fibers or fibers having special anchoring heads at their ends, e.g. headed fibers that are bone-shaped or in the form of a nail with two heads, provide good anchoring but they still suffer from a tendency to clump together when a high percentage of them is incorporated in the matrix. This problem can only be solved by special technological procedures for incorporating the fibers in the matrix, however these procedures increase the cost price very considerably.
Fibers are also known which are provided with hooks and which are presented in the form of plates of stuck-together fibers, thereby considering facilitating incorporation of the fibers in the concrete and mixing thereof. In addition, the hooks significantly improve fiber anchoring. The tips of these fibers diverge in order to prevent them catching one another and clumping together, thereby giving a fiber composite whose surface state includes visible tips.
Discontinuous fibers are also known which are in the form of closed convex loops. U.S. Pat. No. 1,913,707 discloses a fiber in the form of an annular segment whose two ends face one another. With this circular fiber, any increase in friction dissipation obtained by selecting a small radius of curvature occurs to the detriment of anchoring length. Finally, while the cement matrix is being mixed, the segments may open, thereby causing the fibers to clump together.
U.S. Pat. No. 3,616,589 discloses a fiber having a shape which is convex, but closed. The ring may be closed by welding together the two ends of an annular segment. This structure prevents the fibers clumping together, but it does not make it possible to obtain high energy dissipation simultaneously by friction (same problem as with U.S. Pat. No. 1,913,707) and by plastification, with plastification being concentrated to segment portions close to the crack.
U.S. Pat. No. 3,616,589 constitutes the prior art closest to the present invention.